Add empty settings-tabs to htmx layout

This is required if requesting an admin page via HTMx; i do not think it
is done as of now, except for the media picker, that has no layout
because i do not want to update the title.

web/templates/admin/htmx.gohtml

@@ -4,3 +4,4 @@
 -->
 <title>{{ template "title" . }} — Camper</title>
 {{ template "content" . }}
+{{ define "settings-tabs" }}{{ end }}